The catalytic cracking of hydrocarbons is a process which is widely used for producing fuels, in particular for automobiles, which takes place in the vapor phase at a temperature of between 500.degree. and 700.degree. C.
The equipment includes three main elements, namely the reactor, the regenerator and the separator.
The catalyst circulates between the reactor and the regenerator inside transfer tubes. One of them conveys the catalyst to the regenerator, while the other is taken by the regenerated activating agent returning to the reactor.
This catalyst is a solid pulverulent material, in general an aluminosilicate of the zeolite type with a particle size between 20 and 150 .mu.m having a large surface area.
The critical part of this equipment is the regenerator, the base of which includes a perforated metal piece, referred to as the blowing or injection grate. The latter is either flat or curved, which is the most general case.
It is through this grate that a hydrocarbon/ catalyst/air mixture is introduced into the regenerator. The perforations of this grate are equipped with stainless steel nozzles referred to as injectors. At these injectors, acting as venturis, rapid vaporization of the mixture into the regeneration chamber takes place, which gives it a speed of the order of 100 m/s. The catalyst thus entrained at high speed is particularly abrasive. Wear occurs on the injection region and on the top of the grate.
When the mixture enters the regenerator proper, which has a diameter of 3.5 m to 11 m depending on the processing capacity, an abrupt decrease in the flow speed of the gas flow takes place as a result of the change in the passage cross sections, which causes the reaction mixture to be separated into 2 phases. The catalyst remains essentially in the lower part of the regenerator, where it constitutes a layer which is subjected to very strong turbulent movements that are particularly damaging to the covering, but the surface of which remains in steady state, at a constant level just like a liquid.
Because of this abrasion and the temperature prevailing in the regenerator, the top of the injection grate should be equipped with an abrasion-resistant refractory covering which should also withstand the very strong vibrations caused by the turbulence, or else be degraded rapidly.
FR-A-2 569 828 or U.S. Pat. No. 4,651,447 describe a wear-resistant refractory lining formed by anchoring a layer of refractory material, such as a concrete, using special metal elements welded onto a casing to be protected.
This solution requires very long assembly and repair times, and a high degree of erosion/corrosion also remains at the passages. This solution improves the lifetime of the grate, but not the lifetime of the nozzles housed in the passages. Concrete and nozzles must be replaced at each shutdown.
In order to improve this situation, electrofused ceramic nozzles may be substituted in place of the stainless steel nozzles.
The current solution which has to date been most successful therefore consists in jointly using a lining of the abovementioned type and tubular ceramic parts forming nozzles, obtained by melting, casting and molding a mixture consisting essentially of alumina, zirconia and silica. The nozzles are housed in each of the holes in the grate which form the injection passages, and are held in place by a metal collar which also makes it possible to compensate for the differential expansion between the steel grate and each ceramic nozzle.
An assembly of this type is produced in three steps:
fitting the ceramic nozzles, with their metal collar, in the passages in the grate, then welding the metal collar onto the grate, PA1 fitting the metal elements for anchoring the refractory concrete, outside the passage regions, and welding these elements onto the grate, and PA1 pouring the refractory concrete. PA1 Al.sub.2 O.sub.3 =45-51%, ZrO.sub.2 =32-41%, SiO.sub.2 =12-16%, Na.sub.2 O and/or K.sub.2 O32 1-1.2% and the sum MgO+CaO+Fe.sub.2 O.sub.3 +TiO.sub.3 .ltoreq.0.3%.
In the event of partial repair being required, it is difficult to repair only a highly localized region and the repair time is long, which leads to down time for the cracking unit.
The weak point in this current solution therefore remains the concrete which must be replaced fully or partially at each shutdown in order to try to obtain periods of operation of the order of 6 years, while after 9 years of operation only about 10% of the total number of the ceramic nozzles would need to be replaced.
There is therefore a requirement for protection which is more effective and easier to produce than those currently known.